The disclosure of Japanese Patent Application No. 11-214217 filed on Jul. 28, 1999 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a high-pressure fuel pump that pumps fuel from a fuel tank to a high-pressure fuel injection system of an internal combustion engine and regulates the amount of fuel pumped (amount of fuel ejected) by using a spill valve, and also relates to a cam for the high-pressure fuel pump.
2. Description of Related Art
Related high-pressure fuel pumps are described in, for example, Japanese Patent Application Laid-Open Nos. 10-176618 and 10-176619, and the like.
In a typical high-pressure fuel pump of this type, a plunger disposed in a cylinder is reciprocated by a cam that is rotated by an internal combustion engine, as described in the aforementioned laid-open patent applications. During the suction stroke during which a pressurizing chamber defined by the cylinder and the plunger is expanded in capacity, fuel is drawn from a fuel tank into the pressurizing chamber. An amount of fuel drawn into the pressurizing chamber is ejected into a fuel injection passage during the ejection stroke during which the pressurizing chamber is reduced in capacity. During the ejection stroke, the closed valve duration of a spill valve (electromagnetic spill valve) is controlled. A substantive amount of fuel ejected during the ejection stroke is determined in accordance with the closed valve duration of the spill valve controlled during the ejection stroke. That is, while the spill valve is open, fuel pressurized in the pressurizing chamber is allowed to spill into a low-pressure passage even during the ejection stroke. It is not until the spill valve is closed at an appropriate timing during the pressurization of fuel that the fuel ejection into the ejection passage starts. Then, at a timing at which the spill valve is opened again, fuel starts to spill into the low-pressure passage so that the fuel ejection discontinues. By using the spill valve in this manner, the high-pressure fuel pump allows high-precision adjustment of the fuel ejection amount.
During operation of the high-pressure fuel pump, the pressure that is applied to fuel present in the pressurizing chamber as the plunger moves in the chamber-capacity reducing direction during the ejection stroke acts on the spill valve in the valve closing direction. Therefore, when the spill valve is closed at a certain timing during the fuel ejection stroke, the fuel pressure accelerates the closing speed of the spill valve, so that the impact noise produced upon the closure of the valve increases. Particularly during a low-load operation state of the engine, such as an idling operation state or the like, the operational noise produced by the engine is less than during other operational states of the engine, so that the operational noise (impact noise) produced by the high-pressure fuel pump relatively increases to a level that cannot be ignored.
Accordingly, it is an object of the invention to provide a high-pressure fuel pump capable of suitably reducing the operational noise related to the closure of a spill valve even during a low-load operation state of an internal combustion engine, such as an idling operation state and the like.
A first aspect of the invention provides a high-pressure fuel pump having a plunger disposed in a cylinder and which is reciprocated by a cam rotated by an internal combustion. Fuel is drawn from a fuel tank into a pressurizing chamber defined by the cylinder and the plunger during a suction stroke during which a capacity of the pressurizing chamber is increased. An amount of fuel that is regulated based on a control of a closed valve period of a spill valve is ejected from the pressurizing chamber into an ejection passage during an ejection stroke during which the capacity of the pressurizing chamber is reduced. The high pressure fuel pump includes a speed variation device for achieving a smaller changing speed of the capacity of the pressurizing chamber during the ejection stroke than during the suction stroke.
The pressure occurring in fuel in the pressurizing chamber during a movement of the plunger in the capacity reducing direction acts on the spill valve in the valve closing direction, as mentioned above. The magnitude of the pressure acting on the spill valve in the valve closing direction depends on the moving speed of the plunger in the capacity reducing direction, that is, the changing (reducing) speed or rate of the capacity of the pressurizing chamber during the ejection stroke. Therefore, if the changing speed of the capacity of the pressurizing chamber during the ejection stroke is made less than the changing speed of the capacity of the pressurizing chamber during the suction stroke, the pressure acting on the spill valve in the valve closing direction can be reduced and, therefore, the impact noise produced at the time of closure of the spill valve can also be reduced. Such a reduction in the impact noise at the time of closure of the spill valve results in a good reduction in the operational noise of the high-pressure fuel pump during the low-load operation state of the internal combustion engine, such as the idling operation state and the like.
In the high-pressure fuel pump described above, the speed variation means may include the cam. The cam may be constructed so that the cam has an asymmetric cam profile for the ejection stroke and the suction stroke and so that a cam angle for the ejection stroke is greater than a cam angle for the suction stroke.
Due to the cam profile setting that makes the turning angle of the cam during the ejection stroke greater than the turning angle of the cam during the suction stroke, the cam provides a smaller changing speed of the capacity of the pressurizing chamber during the ejection stroke than a cam having a symmetric cam profile for the suction stroke and the ejection stroke. Therefore, the aforementioned operational noise reducing advantage can be achieved easily and reliably.
The cam profile of the cam may also be set so that the changing speed of the capacity of the pressurizing chamber with respect to the cam angle becomes substantially constant during at least a part of the ejection stroke.
The provision of a cam profile portion for a constant changing speed of the capacity of the pressurizing chamber during the ejection stroke brings about a linear change in the amount of fuel ejected. Therefore, in a case where the amount of fuel ejected from the pressurizing chamber is regulated based on a control of the closed valve period of the spill valve, as for example, it becomes possible to perform the closed valve period control in a simplified manner based on a simplified calculation process.
A second aspect of the invention provides a cam for driving a high-pressure fuel pump Having a plunger disposed in a cylinder and that is reciprocated by the cam, which is rotated by an internal combustion engine. Fuel is drawn from a fuel tank into a pressurizing chamber defined by the cylinder and the plunger during a suction stroke during which capacity of the pressurizing chamber is increased. An amount of fuel that is regulated based on a control of a closed valve period of a spill valve is ejected from the pressurizing chamber into an ejection passage during an ejection stroke during which the capacity of the pressurizing chamber is reduced. The cam has a cam profile which is asymmetric for the ejection stroke and the suction stroke, and in which a cam angle for the ejection stroke is greater than a cam angle for the suction stroke.
The adoption of the above-described cam reduces the plunger speed (the changing (reducing) speed of the capacity of the pressurizing chamber) during the ejection stroke, and therefore reduces the operation noise of the high-pressure fuel pump resulting from the impact noise occurring at the time of closure of the spill valve.
In the above-described cam, the cam profile may be set so that the changing speed of the capacity of the pressurizing chamber with respect to the cam angle becomes substantially constant during at least a part of the ejection stroke.
This cam profile allows a simplified control of the closed valve period of the spill valve based on a simplified calculation process.
A third aspect of the invention includes a method of pumping fuel at a high pressure using the structure described above.